A typical data storage system includes a magnetic medium for storing data in magnetic form and a transducer used to write and read magnetic data respectively to and from the medium. A typical data storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute (RPM).
Digital information is typically stored in the form of magnetic transitions on a series of concentric, spaced tracks formatted on the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a number of sectors, with each sector comprising a number of information fields, including fields for storing data, and sector identification and synchronization information, for example.
An actuator assembly typically includes a plurality of outwardly extending arms with one or more transducers and slider bodies being mounted on flexible suspensions. The slider body lifts the transducer head off the surface of the disk as the rate of spindle motor rotation increases, and causes the head to hover about the disk on an air bearing produced by high speed disk rotation. The distance between the head and the disk surface, which is typically less than 1 microinch, is commonly referred to as head-to-disk clearance or spacing.
Within the data storage system manufacturing industry, much attention is presently being focused on reducing head-to-disk clearance as part of the effort to increase the storage capacity of data storage disks. It is generally desirable to reduce the head-to-disk clearance in order to increase the readback signal sensitivity of the transducer to typically weaker magnet transitions associated with the higher recording density written on disks.
In the continuing effort to minimize head-to-disk clearance, manufacturers of disk drive systems recognize the importance of establishing a nominal head flyheight that is sufficient to avoid disk surface defects, such as protruding asperities. As head-to-disk clearances are reduced to achieve additional improvements in disk drive performance, detecting changes in head-to-disk clearance becomes increasingly important. If the clearance becomes too small, then frequent head-disk contact produces head and disk wear which, in turn, generates particles that contaminate the head-disk interface and can lead to a head crash. In addition, lower head-disk clearance results in more thermal asperities for magnetoresistive heads. Thermal asperities can cause data loss by distorting the readback signal to the point that the data is unreadable.
Unexpected changes in head-to-disk clearance of a particular head, which may or may not result in deleterious head-to-disk contact, are generally indicative of a problem with the particular head or head assembly. By way of example, an appreciable decrease in head-to-disk clearance may be indicative of a suspect head.
A number of screening approaches have been developed for use during disk drive manufacturing to identify heads that are flying with insufficient head-to-disk clearance. One approach is to measure the head-to-disk clearance change which occurs during multiple slider airbearing conditions. Methods commonly employed for measuring flying height change as a result of changes in the readback signal include: clearance modulation detector (CMD), clearance change detector (CCD), harmonic ratio flyheight (HRF), and quantitative readback signal (QRS), among others. All of these methods require external instrumentation or physical connection to the output of the arm electronics module to perform the measurement. These methods vary the RPM's of the disks during the testing procedure (i.e., spin-down mode) in order to produce the necessary head-disk clearance measurements during multiple slider airbearing conditions.
A superior method for measuring flyheight change during drive operation, generalized error measurement (GEM), is built directly into the recording channel, and does not require external instrumentation or physical connection to the output of the arm electronics module to perform the measurement. GEM directly measures various magnetic parameters of the head and disk, as well as figures of merit for the channel electronics. The GEM circuit monitors head flyheight on all data surfaces, channel noise, signal coherence, signal amplitude, writing parameters and other operational characteristics. Unlike conventional error monitors, GEM provides for direct detection of specific mechanisms that can precede a disk drive failure. Unfortunately, GEM measurements cannot be made by varying the RPM's of the disks during pre-shipment testing, as utilized by earlier methods. Newer designs of airbearing sliders lift off the disk surface at increasingly lower RPM's, greatly increasing the risk of head damage during spin-down types of testing.
There is a need for a disk drive reliability test which may be employed by a disk drive manufacturer prior to customer shipment of the drive for estimating the long-term reliability of the head-disk interface. This test should not require any external instrumentation or physical connections to the output of the arm electronics module, and should not require the potentially dangerous technique of varying the RPM's of the disks during the testing process to generate multiple slider airbearing conditions.